1. Field of the Invention
This invention relates generally to earth boring, especially to improvements to bits having cutters rotatably secured to bearing shafts.
2. Background Information
The conventional contemporary way to manufacture an earth boring drill (rock) bit is to forge a body of low alloy steel to a finished shape that includes a rough bearing envelope, and then to machine an integral bearing shaft or pin to a final shape. The resulting head section (of which there are usually three) is then carburized and heat treated to provide the requisite strength and wear resistance to the bearing shaft, while leaving the supporting leg with the desired ductility and strength.
Machining of the bearing shaft is considered relatively difficult due to the complex shape of a head section, and selective heat treatment to harden primarily the bearing area is troublesome, often requiring sectionalized copper plating and subsequent stripping.
Seemingly, it would be attractive to manufacture the bearing shaft as a separate unit--if a practical and strong method could be found to replace the strong and proven integral structure favored now by bit manufacturers.
Indeed, the concept of manufacturing the bearing as a separate unit is not new, and may be seen in the U.S. Pat. No. 1,803,679 of F. L. Scott, "Drill Cutter Retainer Means", May 5, 1931. Here, the bearing sleeve has a conical exterior and is secured to a threaded bearing lug. A resilient retainer ring of polygon cross section, located radially above the threads on the lug and inside the bearing sleeve, is used to retain the cutter on the bearing shaft. Another arrangement using a threaded lug and sleeve may be seen in one of the early rock bits, U.S. Pat. No. 1,320,384, which used a threaded retainer ring to retain the cutter on a bearing lug and sleeve combination. Other examples of bearing sleeve constructions may be seen in U.S. Pat. Nos. 3,998,500, 4,266,622 and 4,235,295. In addition there are being sold commercially by Reed Tool Company of Houston, Texas bits using a forged and machined lug welded to cast bit bodies.
Further, Hughes Tool Company, the assignee of applicants, has manufactured and sold a large diameter bit which has a bearing sleeve attached to a bearing lug by the shrink fit process after aligning the passageways in the sleeve and the leg of the bit to enable retention of the cutter with what is known as the "ball lock" method.
With the introduction of electron beam welding to rock bit manufacture, attention has turned to this method of attaching a bearing shaft to the leg of a bit, as may be seen in U.S. Pat. Nos. 4,043,411 and 4,127,043.
And yet, the more popular sizes of rock bits still use today the integral bearing pin and leg, which has been the standard in the industry since the 1930s. Over fifty years of usage has proven this to be the best and most reliable method of forming a bearing on the shaft of a rock bit.